If you’re asking the question as a guarantee into the future, there truly isn’t any absolute certainty. Everyone has agency. That was part of the reason that King Mosiah in the Book of Mormon wanted to set up a system of judges. None of his sons could or would take the kingdom, and even though they repented and turned out to be incredible missionaries, even Mosiah recognized that the right temptation could possibly cause them to fall.
On the other hand, if you’re asking how can I know that John has experienced the “mighty change of heart” spoken of in the Book of Mormon and has truly repented and forsaken his former sins, you’re in luck — I could actually take 4 or 5 posts to cover it thoroughly. I’m going to try to condense it down to one.
Repentance v. “Serving Time”
This was my first concern as my then “romantic interest” began unfolding his story to me. I knew my first husband was a con artist, and I knew I was susceptible to being deceived. That was the beginning of learning the difference between repenting serving time.
For instance, if a woman met a man at the gym and allowed the relationship to develop into a full-blown affair, repentance is obviously needed. If, after the affair comes to the knowledge of her husband and bishop, she attends minimal church services, returns to the exact same friends at the gym who supported the illicit relationship and thought it was cute, and continues to take far more time than is necessary on her workout mornings, it is hard to believe that she is doing anything but “serving her time.”
On the other hand, if the guilt of the sin were to move her to accept her responsibility for the mess she has made in many people’s lives, confess willingly her sins to her husband and bishop, begin a renewed effort of scripture study, prayer, family home evening, etc., change gyms, and become highly accountable to her husband for her time, the evidence points to true repentance.
This pattern is outlined in scripture:
By this ye may know if a man repenteth of his sins—behold, he will confess them and forsake them. — Doctrine and Covenants 58:43
The scriptures also say
Cursed is he that putteth his trust in man, or maketh flesh his arm, or shall hearken unto the precepts of men,save their precepts shall be given by the power of the Holy Ghost. — 2 Nephi 28:31
Fortunately, the power of repentance is real, and if we are seeking to repent and live life on God’s terms, we have many, many promises we can claim!
Ancient and modern prophets have all witnessed of God’s great power. Here are a few:
42 Behold, he who has repented of his sins, the same is forgiven, and I, the Lord, remember them no more.
43 By this ye may know if a man repenteth of his sins—behold, he will confess them and forsake them. — Doctrine and Covenants 58.
24 He doeth not anything save it be for the benefit of the world; for he loveth the world, even that he layeth down his own life that he may draw all men unto him.Wherefore, he commandeth none that they shall not partake of his salvation.
27 Hath he commanded any that they should not partake of his salvation? Behold I say unto you, Nay; but he hath given it free for all men; and he hath commanded his people that they should persuade all men to repentance.
28 Behold, hath the Lord commanded any that they should not partake of his goodness? Behold I say unto you,Nay; but all men are privileged the one like unto the other,and none are forbidden. — 2 Nephi 26.
Evidence of a Mighty Change of Heart
Again, more evidence from ancient and modern prophets:
51 They are they who received the testimony of Jesus, and believed on his name and were baptized after the manner of his burial, being buried in the water in his name, and this according to the commandment which he has given—
53 And who overcome by faith, and are sealed by the Holy Spirit of promise, which the Father sheds forth upon all those who are just and true. — Doctrine and Covenants 76.
And now, verily, verily, I say unto thee, put your trust in that Spirit which leadeth to do good—yea, to do justly,to walk humbly, to judge righteously; and this is my Spirit. — Doctrine and Covenants 11:12
I will be the first to admit that I personally struggle with judging righteously. It’s an outgrowth of my own weaknesses and living so long in a world where I had to fight to retain a sense of being a human being and having worth. When I feel attacked, I still tend to react by wanting to attack back.
Even so, it is important to remember that we are called to judge righteously. We have to look — not at the past — but at the present. What are the evidences in a person’s life? Are we reacting out of fear? Is it fair to hold one piece of evidence up against years of habits and patterns?
The answers change from person to person. I can testify, though, that if the condition of a person’s heart affects your ability to watch over your stewardships, you can ask God and receive the answer you need.